The principles of sensory reception and motor function are being studied, with Salmonella typhimurium, as the model system. This bacterial species, like many others, is motile and migrates in response to chemical gradient stimuli in its environment. The motor organelle is the flagellum, a very thin helical filament which rotates to provide thrust. In fact, Salmonella possesses a number (typically 5-10) of these flagella which form a bundle during translational movement. Intermittently, the bundle flies apart and the cell "tumbles". These tumbles are essential since it is regulation of their occurrence in response to gradient stimuli that produces chemotactic migration. We are presently exploring the nature of tumbling by high intensity dark field microscopy and attempting to get micrographs of the individual filaments as they function in motility. Future work will include an examination of motility in relationship to the state of the cytoplasmic membrane.